Level control mechanism for road building machines



06h 1967 J. CURLETT ETAL LEVEL CONTROL MECHANISM FOR ROAD BUILDING MACHINES Filed Sept. 16, 1964 2 Sheets-Sheet l INVENTOR. JOHN CURLETT RAYMOND A. QUERIES 92 A PA TENT AGE/V7 0d. 17, 1967 J. CURLETT ETAL 3346,5976

LEVEL CONTROL MECHANISM FOR ROAD BUILDING MACHINES Filed Sept. 16, 1964 2 Sheets-Sheet 2 INVENTOK JOHN CURLETT mama/v0 A. GURR/ES PATENT AGE/VT United States Patent Ofitice 3,346,976 Patented Oct. 17, 1967 3,346,976 LEVEL CONTROL MECHANISM FOR ROAD BUELDING MACHINES John Curlett, Los Gates, and Raymond A. Curries, San Jose, Caliii, assignors to Gurries Manufacturing Co., San Jose, Calif., a corporation of California Filed Sept. 16, 1964, Ser. No. 396,827 16 Claims. (Cl. 37-156) ABSTRACT GF THE DISCLOSURE A level control mechanism for the blade or moldboard of a road grader or the working implement of similar machines which controls automatically the longitudinal disposition of the main blade-suspending frame of the machine relative to an external grade reference to maintain a predetermined relative disposition therebetween and thus maintain the entire blade at its desired operational level regardless of its shear angle and the existent ground level encountered by the machine.

The present invention relates generally to road building machines, and more particularly, to a level control mechanism for the workin implement of a road building machine, such as, for example, the earth-moving blade of a road grader.

The level control mechanism of the present invention can be advantageously applied, by way of example, to a conventional road grader which generally includes an elongated, arched frame that is supported fore and aft by suitable wheels and adjustably suspends at an intermediate position a generally transversely-extending earth -moving blade. More particularly, the blade is suspended for vertical adjustment, lateral adjustment, and angular adjustment, as required and as controlled by the machine operator. However, severe problems in maintaining a requisite longitudinal grade and cross-slope are encountered. By way of example, if earth is being moved by a blade positioned at a shear angle of 45 relative to the direction of machine motion (a typical operation), if the ground level rises in front of the machine, the front wheels of the grader will rise so that blade height will increase. However, the longitudinal slope of the angularly-disposed blade will also change. Under operator control, the blade can be lowered so that its central portion may be reset at the desired grade level, but because of the change in the longitudinal blade slope, the forwardly projectin-g end of the angularly-disposed blade will be higher than the rearwardly projecting end thereof and additional correction is required. The multiplicity of requisite corrections is difficult for even the most experienced operator.

In view of this and other operational defects in existent road graders and other road building machines wherein longitudinal grade and crossslope must be accurately maintained, it is a general object of the present invention to provide a level control mechanism which automatically maintains the blade or other working implement of a road building machine at a desired level both longitudinally and transversely relative to the direction of machine motion.

More particularly, it is a significant feature of the present invention to provide a level control mechanism that can be applied to existent road graders Or other road building machines with minimal modification thereof.

Additionally, it is a feature of the invention to provide a level control mechanism which automatically functions to maintain the blade or other working implement at a desired height relative to a grade wire, existent surface, or any other pre-established external grade reference.

Yet more particularly, it is a feature to provide a level control mechanism including adjustment means connected between the front supporting wheels of a road grader and the forward portion of the frame whereby such frame and the blade suspended therefrom are raised or lowered in response to vertical deviations relative to the aforementioned external grade reference so as to automatically maintain the frame and supported blade in a predetermined longitudinal disposition relative to such grade reference.

Such adjustment means, in accordance with another feature of the invention, preferably takes the form of a pair of hydraulic cylinders connected respectively to two transversely spaced wheels at the front end of the road building machine, such cylinders being hydraulically associated to automatically accommodate differences in elevation of the surface encountered by such wheels.

A related feature of the invention is the arrangement for mounting a grade reference sensor in transverse alignment with the blade to assure accurate control of blade height.

In accordance with another aspect of the invention, it is a feature to provide a level control mechanism for a road building machine including means for automatically adjusting the transverse slope of the blade or other working implement so as to maintain the requisite pre-established crossslope of the graded surface.

Preferably, it is a feature of the invention to control the transverse slope of the blade or other working implement in response to a gravity sensing device such as a pendulum.

Yet more particularly, it is a feature to provide means including a gravity responsive device for controlling the transverse slope of the working implement which device is mounted on the main frame of the unit and is operatively interconnected with the adjustably-mounted working implement by feedback linkage whereby further change in disposition of the working implement is stopped upon attainment of a pro-established cross-slope setting.

It is an additional feature of the invention to provide a level control mechanism including a cross-slope control for the working implement involving the simple addition of a hydraulic ram or other extensible member to the existent vertical control linkage for the working implement.

These as well as other objects and features of the invention will become more apparent from a perusal of the following description of an exemplary embodiment of the present invention depicted in the accompanying drawings wherein:

FIG. 1 is a perspective view of a generally conventional road grader having a level control mechanism applied thereto in accordance with the present invention, the road grader being viewed from the left front thereof,

FIG. 2 is an enlarged fragmentary elevational view of the front portion of the road grader,

FIG. 3 is a somewhat diagrammatic elevation of the right side of the road grader with the control mechanism incorporated therein,

FIG. 4 is a diagrammatic representation of the grade control system of the mechanism, and

FIG. 5 is a diagrammatic view of the cross-slope control mechanism.

As shown in FIGS. 1 and 3, the road grader to which the level control mechanism of the present invention is applied, is of generally conventional design including a rigid frame 10 mounted at its rearward end on dual wheel units 12 that are driven from a conventional diesel engine 14 mounted thereabove. The frame 10 extends forwardly in a somewhat arched configuration for support at its forward extremity on transversely spaced wheels 16, 16', which can be steered about vertical spindles, one of which is shown at 17 in FIG. 1, through suitable interconnecting linkage to a steering wheel 18 mounted above the rear portion of the frame in front of the operators seat 20.

An auxiliary blade-mounting frame 22 is pivotally secured as indicated at 24 to the underside of the forward end of the main machine frame and extends rearwardly to rotatably support a quadrant 26 to the underside. of which the earth-moving blade 30 is rigidly attached.

Conventional drive linkage 32 is connected between the quadrant 26 and a control handle adjacent the operators seat to provide rotative adjustment of the quadrant and consequent angular adjustment of the blade about a substantially vertical axis. To raise or lower the blade 30, links 34, 34 are pivotally secured to opposte sides of the auxiliary frame 22 at their lower ends and are pivotally connected to the extremities of crank arms 36, 36 at their upper ends. The crank arms 36, 36 are rotated, either indviidually or simultaneously, from suitable actuating lirfl age 38, 38' controlledby the operator so that the auxiliary frame 22 may be raised or lowered to, in turn, effect vertical adjustment of the blade 30 or a variation in the transverse slope thereof, the pivotal mounting 24 of the auxiliary frame 22 permitting such adjustments. To effect lateral motion of the blade 30, additionallinkage, indicated at 40, and also under the control of the operator, effects lateral shifting of the auxiliary frame 22 and the blade 30 therewith. No further details of the road grader will be explicated since the described construction is conventional and in and of itself forms no part of the present invention. As a corollary, it will be apparent from the following description of the invention that application thereof to road graders or other road building machines differing in details from the illustrated structure is within the contemplated scope of the invention.

In accordance with the present invention, the conventional axle joining the front wheels 16, 16' is replaced by two centrally conjoined parallelogram units which are pivoted in mirror like symmetry from a horizontal pin 44 fixed to the front of the frame 10, thus to allow each wheel individually to be raised or lowered relative to the supported frame and also enable the front wheels 16, 16 to be leaned laterally.

With added reference to FIG. 2, each parallelogram unit 42 includes a main axle section 45 pivotally mounted at its ends on the fixed pin 44 and another pin 46 at the lower end of a support bracket 47 for the wheel spindle 17. A rod 48 is pivotally connectedat its opposite ends to a pin 49 at the upper end of the bracket 47 and a movable pin 50 on the upper end of a link 51 pivoted at its lower end on the fixed frame pin 44, thus completing the parallelogram unit 42. Consequently, each of the parallelogram units is joined to the frame 10 at the common pin 44 but can be shifted laterally in a conjoint fashion to provide lateral leaning of the wheels 16, 16' while maintaining parallelism thereof or can be individually raised or lowered to provide vertical adjustment of the wheels 16, 16 relative to the frame 10, again while maintaining wheel parallelism.

To actuate lateral leaning of the wheels 16, 16, a double-acting hydraulic ram 52 is connected pivotally between the upper bracket pin 49 and a horizontal pin fixed on the frame, this pin being aligned with the movable pin 50 when the wheels 16, 16' are in their upright dispositions, as best shown in FIG. 2. Suitable hydraulic lines indicated at 53 effect, under operator control, extension or retraction of the ram 52 to thus shift the rods 48, 48' thereby changing the disposition of both parallelogram units 42, 42' and the amount of wheel lean.

For raising or lowering of the wheels 16,16, a doubleacting hydraulic ram 54, 54' is pivotally connected in an angular disposition between each side of the forward portion of the frame 10 and the bracket pin 46. Accordingly, if both. rams 54, 54' are simultaneously extended, the front end of the frame 10 will be raised relative to the supporting wheels 16, 16. On the other hand, if the rams are retracted, the front end of the frame 10 will be lowered relative to the wheels 16, 16'. Since the wheels 16, 16' are connected to the frame 10 by the parallelogram units 42,

4 42', the planes defined by the wheels will remain in parallelism regardless of their elevation relative to the supported frame.

The two height-control rams 54, 54 are simultaneously actuated through common feed and return conduits 56,.

The hydraulic control valve 66 is actuated by a control member that is operatively associated with an external grade reference in a manner such that the earth-moving blade .30 is maintained at a predetermined height relative to such reference. More particularly, as shown mostclearly in FIG. 1, a bracket is rigidly secured to one side of the auxiliary blade-mounting frame 22 to project laterally therefrom in tratnsverse alignment with the blade 30 and mount at its outer end the aforementioned control valve 66 in mechanically cooperative relationship with a sensor member in the form of an outwardly projecting feeler fork 82 that is arranged to engage a grade wire W strung in parallelism to the path of machine motion at a desired level, suitable supporting stakes S for the wire W being disposed at spaced intervals along such path. Preferably, the valve 66 and the associated feeler fork 82 are constructed in a manner described in detail in US. Patent No. 3,142,915, so as to perform the functions already indicated and also provide for manual adjustment of the level of the feeler fork 82 through rotatable mechanical rods 84 whose. extremity is connected to a rotary handle 86 adjacent the operator's seat 20.

Once the operator has set the blade 30 at a predetermined level by the appropriate manipulation of the vertical blade control linkage 34, 36, 38 and has also adjusted the feeler fork 82 in encompassing relationship with the external grade wire W, the blade height will be automatically maintained at such predetermined level throughout the grading operation. More particularly, as the road grader advances and if, for example, the front wheels 16, 16' encounter a rise in the surface being graded, the resultant raising of the wheels effects a raising of the main frame 10 and also the auxiliary frame 22 together with the feeler fork 82 mounted therefrom. The feeler fork movement is transmitted to the described control valve 66 in a fashion to deliver fluid through conduits 56, 58, 58' to the lower ends of the hydraulic rams 54, 54 to effect retraction thereof and thus lower the frame 10. Such lowering of the main frame 10 continues until it, the auxiliary frame 22, blade 30 suspended therefrom, and the feeler fork 82 regain the pre-established level, whereupon the valve 66 closes and further vertical motion of the frame 10 and the blade 30 mounted therefrom ceases. If, on the other hand, the'front wheels encounter a depression, the feeler fork 82 moves in a fashion so as to open the control valve 66 to supply fluid under pressure through conduits 56, 60, 60' to extend the rams 54, 54' and, in turn, raise the frame 10, the blade 30 supported therefrom, and the feeler fork 82 until the latter has regained its original level, whereupon the vertical lifting ceases. Since the feeler fork 82 is disposed immediately transversely of the blade 30, the accuracy of the blade height is maintained precisely. Furthermore, since the front wheels 16, 16. encounter either rises or depressions in the surface to be gradedin advance of the blade 30, the changes are anticipated and corrections are made before the blade encounters such rise or depression. In short, the longitudinal disposition of the frame 10 is maintained in constant preset relation to the grade wire W regardless of the ground level encountered by the advancing machine. Consequently, the blade 30 is also maintained at the desired level regardless of its shear angle.

If but one of the front wheels 16, 16' encounters a rise or depression, a pressure imbalance is established between the two hydraulic rams 54, 54', and since they are connected through the branch conduits 58, 58' and 60, '60, such pressure imbalance is automatically accommodated so that no variation in transverse angular disposition of the frame or the blade supported therefrom is experienced. If such lifting (or lowering) of one wheel 16 or 16' changes the general height of blade disposition, the feeler fork 82 will, of course, actuate the control valve 66 in the manner described hereinabove to automatically readjust the blade height to the requisite grade level.

Generally, since the rear wheel units 12 of the grader pass over surfaces which have already been encountered by the blade 30 and are accordingly graded to the desired finished surface, no lifting nor lowering of the rear end of the machine frame will be experienced. However, if, for any reason, such lifting or lowering of the rear end of the frame were experienced, the feeler fork 82 would again be shifted to actuate the valve 66 and adjust the blade to the requisite pre-established height.

It is to be particularly noted that minimal modification of a conventional road grader is necessary to provide the described grade control. A simple modification of the mounting of the front wheels 16, 16' to enable individual vertical adjustment thereof is the only change required. The remaining elements of the grade control mechanism constitute simple additions of hydraulic and mechanical elements which in no way interfere with the ordinary operation of the grader. While the described double-acting rams 54, 54 are preferred to give positive height control, it will be obvious that single acting cylinders can be substituted therefor, pressure being applied to raise the frame and relieved to permit frame lowering under its own weight.

In accordance with another aspect of the invention, an additional, simple modification of a conventional road grader of the general type illustrated together with the addition of a gravity responsive control means enables automatic control of the transverse slope of the earth-moving blade 30 and the cross slope of the graded surface. More particularly, for this purpose, one of the vertical adjustment links 34 connected to the auxiliary frame 22 for raising or lowering that side portion of the blade 30 is modified to incorporate a hydraulic ram 90 which normally will function after the fashion of the replaced rigid link when not actuated, but when actuated in a manner to be described hereinafter, will raise or lower that side of the auxiliary blade-mounting frame 22 and the blade 30 mounted therefrom to provide for an adjustment of the transverse slope of the blade to re-establish a desired transverse disposition thereof, as set by the operator.

With reference to FIGS. 1 and 3, and additional reference to the schematic diagram of FIG. 5, such hydraulic ram 90 constitutes a double-acting ram connected in a hydraulic circuit including feed and return conduits 92, 94 which are placed in communication with a hydraulic pump 96 and reservoir 98 through a four way valve 100 which is controlled by a gravity responsive mechanism. Preferably, the valve 100 and the gravity-responsive mechanism take the form of the pendulum controlled, valve actuating mechanism 102 shown in our Patent No. 3,296,- 722 to which reference is made for details of construction and operation. Preferably such pendulum controlled, valve actuating mechanism 102 constitutes a unit that is secured rigidly to the frame adjacent the operators seat and which by means of a rotatable control rod 104 having a handle 106 accessible to the operator can be preset to a deenergized position at a predetermined crossslope. When the angular transverse disposition of the road grader changes from the pre-established value, the pendulum indicated diagrammatically at 108 in FIG. 5 swings to one side or the other under gravitational force 6 to, in turn, open the control valve 100 in a fashion to supply hydraulic fluid under pressure through conduits 92 or 94 to the lower or upper end of the double-acting hydraulic ram which will, in turn, lower or raise the lateral portion of the blade 30 thereunder.

In order to de-energize further ram actuation when the requisite cross-slope of the blade 30 has been attained, feedback means from the blade '30 to the pendulum-controlled, valve-actuating mechanism 102 is provided. More particularly, a telescopic rod 110 is operatively connected at one end to a member 112 mounted on the auxiliary blade-mounting frame 22 so as to be rotated in one direction or the other in response to any variation in transverse blade disposition. The rod 110 projects forwardly and upwardly and at its remote end is connected to a step-up gear box 114 mounted on the main frame and the amplified output from such gear box is transmitted by the ro-.

tation of another rod 116 extending rearwardly under the main frame for connection to the pendulum-controlled, valve actuating mechanism 102 to provide a feedback input in the manner described in detail in US. Patent No. 3,296,722. In order to accommodate relative motions between the main frame 10 of the grader and the auxiliary frame 22 which mounts the blade 30, suitable universal, pivotal connections are provided at each end of the feedback rod 110 extending therebetween and the rod is of the described telescopic construction to enable variation in its length.

The operation of the cross-slope control can be briefly stated. If, for example, the left side of the machine is raised by the surface encountered, the left side of the blade 30 will also rise, but the disposition of the pendulum 108 will change at the same time to instigate movement of the valve to supply hydraulic fluid under pressure through conduit 94 to the lower end of the hydraulic ram 90 effecting retraction thereof and a consequent raising of the right side of the blade 30. When the original transverse slope of the blade 30 has been re-established, such information is transmitted by rotation of the feedback rods 110, 116 to the pendulum control unit and effect closure of the valve 100 so that no further change in the transverse slope of the blade will be experienced. Obviously, at the same time that such change in cross-slope is being accommodated, the rise on the left side of the machine will effect movement of the feeler fork 82 to accordingly exert fluid pressure at the lower ends of the hydraulic rams 54, 54' mounting the front wheels 16, 16' so that the frame 10 will be lowered until the original level is reestablished. In short, through the cooperation of the described grade control operative from a pre-established external reference and the gravity-responsive cross-slope control, the earth moving blade 30 will always be maintained in the originally established plane of travel.

Obviously, various modifications and/ or alterations can be made in the described structure without departing from the spirit of the invention, and the foregoing descrip tion of an exemplary embodiment is provided only for illustrative purposes and is not to be constructed in a limiting sense. The actual scope of the invention is to be indicated only by reference to the appended claims.

a What is claimed is:

1. Level control mechanism for the working implement of a road building machine including a frame, mobile support means therefor, and separate means for suspending laterally-spaced portions of the working implement from the frame, said control mechanism comprising means connected bet-ween the mobile support means and the frame for vertically adjusting the disposition of the frame, means including a member adapted for operative association with an external grade reference for controlling vertical adjustment of the frame on the mobile support means, means connected between the frame and at least one laterally-spaced portion of the working implement for vertically adjusting that portion of such working implement, and means responsive to a lateral deviation in the transverse slope of the working implement for controlling vertical adjustment of the laterally-spaced portion of the working implement.

2. Level control mechanism according to claim 1 wherein said implement-adjusting means includes a hydraulic ram.

3. Level control mechanism according to claim 2 wherein said adjustment-control means includes a pendulum-actuated hydraulic control valve connected to control the supply .of hydraulic fluid to said hydraulic ram.

4. Level control mechanism according to claim 3 wherein said pendulum-actuated hydraulic control valve is mounted on said frame and which comprises feedback means connected between the working implement and said pendulum-actuated hydraulic control valve to close said valve upon attainment of a pre-established transverse slope of the working implement.

5. Level control mechanism for the working implement of a road building machine including a frame having transversely-spaced supporting wheels, and separate means for suspending laterally-spaced portions of the working implement from the frames, said control mechanism comprising means connecting the transversely-spaced supporting wheels to the frame for individual relative vertical adjustment, means for actuating relative vertical adjustment of the wheels simultaneously, means including a member adapted for operative association with an external grade reference for controlling the vertical adjust-ment of the wheels relative to the supported frame, means connected between the frame and at least one laterally-spaced portion of the working implement for vertically adjusting that portion of such working implement, and means responsive to a lateral deviation in the transverse slope of the working implement from a preestablished value for controlling vertical adjustment of the laterally-spaced portion of the working implement.

6. Level control mechanism according to claim 5 wherein said Wheel-adjustment actuating means includes hydraulic rams extending downwardly from opposite sides of the frame for connection to the supporting wheels.

7. Level control mechanism according to claim 6 wherein said wheel adjustment actuating means includes a source of hydraulic pressure connected to said hydraulic rams through a common conduit.

8. Level control mechanism according to claim 6 wherein said wheel connecting means includes a shiftable parallelogram unit connected between the frame and each of the transversely-spaced wheels so that vertical motion and lateral leaning of the wheels is accommodated.

9. Level control mechanism according to claim 8 which comprises an additional hydraulic ram connected in substantially a horizontal disposition between said frame and one of said parallelogram units to effect, upon actuation, lateral leaning of the wheels.

10. Level control mechanism for the blade of a road grader including a frame having transversely-spaced supporting wheels at one end, additional mobile supporting means at the remote end, and separate means for suspending laterally-spaced portions of the blade therefrom, said control mechanism comprising means connected between said transversely-spaced supporting wheels and the frame for vertically adjusting the end of the frame supported thereby, means including a member adapted for operative association with an external grade reference for controlling vertical adjustment of the frame on the supporting wheels, means connected between the frame and at least one laterally-spaced portion of the blade for vertically adjusting that portion of the blade, and means responsive to a lateral deviation in blade slope for controlling vertical adjustment of that portion of the blade.

11. Level'control mechanism for the working imple- 8 ment of a road building machine including a frame having transversely-spaced front and rear supporting wheels and means for suspending the working implement from the frame, which control mechanism comprises separate,

frame adjusting means connected Between said frame and the front supporting wheels for varying the vertical distance therebetween, and means including a control member adapted for operative association with an external longitudinally-extending grade reference for simultaneously actuating said separate frame adjusting means in response to a deviation of working implement height from a pre-established relationship to the external grade reference so as to reestablish said frame longitudinally in its original relation to the external grade, reference.

12. Level control mechanism according to claim 11 wherein said frame adjusting means includes a hydraulic ram connected between the frame and each front supporting wheel.

13. Level control mechanism according to claim 12 wherein said actuating means for said hydraulic rams includes a source of hydraulic fluid under pressure, said hydraulic rams being connected to said pressure source through a common hydraulic conduit.

14. Level control mechanism for the working implement of a road building machine including a frame having transversely-spaced supporting wheels, which control mechanism comprises a shiftable parallelogram unit connected between the frame and each of the transverselyspaced wheels so that vertical motion and lateral leaning of the wheels is accommodated, means operatively connected between the frame and said parallelogram unit to laterally shift the same andeffect lateral leaning of the wheels, and means connected between .the frame and each of said parallelogram units to effect vertical pivotal motion thereof to raise or lower the wheels relative to the frame.

15. Level control mechanism for the working implement of a road building machine including a frame having mobile support means and carrying the working implement which level control mechanism comprises frameadjusting means connected between said frame and said mobile support means for varying the vertical angular relation therebetween in the direction of machine travel and means operatively associated with an external grade reference defining a rectilinear path extending generally along the path of machine travel for actuating said frame adjusting means to maintain the longitudinal disposition of said framein substantially constant vertical angular relation to such rectilinear path.

16. Level control mechanism according to claim 15 which comprises means adjustably supporting laterallyspaced portions of the working implement from said frame, and means operative in response to cross-slope deviation of the working implement from a predetermined disposition for actuating said adjustable support means to return the working implement to the original predetermined disposition.

References Cited UNITED STATES PATENTS 1,452,557 4/1923 Harroun -41 1,875,778 9/1932 Taylor 37157 2,870,555 1/1959 Gurries et al. 371'80 2,883,777 4/1959 Gurries et a1. 37-180 2,941,319 6/1960 Beemer et al. 37-156 2,952,245 9/1960 Curlett et al. 37144 X 2,961,783 11/1960 Bowen et al. 37-156 3,126,653 3/1964 Bourgeous.

3,142,915 8/1964 Curlett et al. 37180 3,229,391 1/ 1966 Breithbarth et al. 37-4156 ABRAHAM G. STONE, Primary Examiner.

R. L. HOLLISTER, Assistant Examiner. 

15. LEVEL CONTROL MECHANISM FOR THE WORKING IMPLEMENT OF A ROAD BUILDING MACHINE INCLUDING A FRAME HAVING MOBILE SUPPORT MEANS AND CARRYING THE WORKING IMPLEMENT WHICH LEVEL CONTROL MECHANISM COMPRISES FRAMEADJUSTING MEANS CONNECTED BETWEEN SAID FRAME AND SAID MOBILE SUPPORT MEANS FOR VARYING THE VERTICAL ANGULAR RELATION THEREBETWEEN IN THE DIRECTION OF MACHINE TRAVEL AND MEANS OPERATIVELY ASSOCIATED WITH AN EXTERNAL GRADE REFERENCE DEFINING A RECTILINEAR PATH EXTENDING GENERALLY ALONG THE PATH OF MACHINE TRAVEL FOR ACTUATING SAID FRAME ADJUSTING MEANS TO MAINTAIN THE LONGITUDIANL DISPOSITION OF SAID FRAME IN SUBSTANTIALLY CONSTANT VERTICAL ANGULAR RELATION TO SUCH RECTILINEAR PATH. 